1. Field of the Invention
This invention relates to a voltage regulator and, in particular, to a voltage regulator that provides enhanced resolution over a larger range of voltage while also achieving improvements in size, cost and reliability.
2. Discussion of Related Art
Voltage regulators are provided to maintain voltage levels at a predetermined level or within a predetermined range. Voltage regulators are used to protect devices used, for example, in broadcast transmission and telecommunications, computers, manufacturing and industrial control by insuring that voltage provided from a power source to a load is within the operating limits of the load.
There are a variety of known voltage regulators. One conventional voltage regulator includes a buck boost transformer disposed between a source and a load. The buck-boost transformer has a secondary winding in series with the line voltage source. Voltage to the primary winding of the buck-boost transformer is controlled by a variable autotransformer resulting in the addition or subtraction of voltage from the line voltage. A movable brush in the autotransformer is controlled by a motor which itself is controlled by a controller monitoring the output voltage. This conventional design is relatively complicated and has a large number of moving parts that limit the reliability of the device. Further, the device has a relatively slow response time to changes in voltage and the components must be sized to several times the corrective power rating of the load.
An improvement to the above-described device integrates the buck-boost transformer and autotransformer. In this device, a first part of the autotransformer winding is in series with the line voltage. A second part of the winding determines the total corrective range the first part can provide and the connection point in this first part of the winding determines what percentage of this corrective voltage that can be added to and what percentage can be removed from the line voltage. The brush of the autotransformer is placed in series between the source and load and is again controlled by a motor operating under the control of a controller monitoring the output voltage. Although this integrated device results in a slight reduction in the number of components, the device still suffers from a relatively complicated design, moving parts and slow response time and the components must again be sized to several times the corrective power rating of the load.
Another conventional voltage regulator is a ferro-resonant transformer. In this device, the magnetic core of the transformer is substantially saturated. A secondary winding draws power from a primary winding to replace power delivered to the load. Because of the saturation of the core, however, the impact of any change in voltage across the primary winding on the voltage across the secondary winding is limited thereby allowing the secondary winding to maintain a relatively constant voltage. Another secondary winding forms part of a resonant tank circuit which helps to maintain saturation of the core. This device suffers from relatively large audible noise, is relatively inefficient thereby leading to the production of excess heat, and creates distortions in the voltage waveforms. The device also is current limiting and must also be sized to handle the maximum power of the load. Further, the device is susceptible to line frequency changes because it is designed for a fixed frequency. The device also can suffer from poor resolution in the output voltage.
Another conventional voltage regulator includes an autotransformer or isolation transformer in which taps on the transformer are selected in response to semiconductor switches. These devices generally suffer from poor output resolution because increased resolution requires additional switches which then increase the cost of the device and reduces the reliability of the device. The switches are particularly susceptible to damage from overload conditions or load transients because they are located in the circuit between the source and load. The device must therefore again be sized to handle the maximum power of the load.
Another conventional voltage regulator functions in a manner similar to the ferro-resonant transformer. In this device, an additional control winding is supplied with direct current to cause saturation of the transformer core. Saturation of the core enables the secondary winding to maintain a substantially constant voltage despite variations in the line voltage on the primary winding. This device, however, again creates distortions in the voltage waveforms (requiring significant filtering to reduce these distortions), is relatively inefficient, and must be sized to handle the maximum power of the load.
Yet another voltage regulator includes a plurality of buck-boost transformers having secondary windings disposed in series between the source and load. Delivery of current to the primary windings of the transformers is controlled such that each transformer is capable of adding to, or subtracting from, the line voltage. Each transformer is configured to generate the same voltage level change between the source and the load. As a result, the device is incapable of achieving sufficient voltage resolution over a large voltage range. For example, if the transformers were configured to generate a voltage level change with relatively high resolution (e.g., 1%), ten separate transformers would be required to achieve only a 10% range. Similarly, if the transformers were configured to permit adjustment over a large voltage range, either the resolution of the transformers would need to be reduced to limit the number of transformers (e.g., four transformers at 5% resolution for a 20% range) or a large number of transformers would have to be added (e.g., twenty transformers at 1% resolution for a 20% range) increasing the size and cost of the device and reducing its reliability. This device is also sensitive to changes in power requirements for different loads thereby requiring recalibration of switching elements.
Yet another conventional voltage regulator includes a plurality of buck-boost transformers having secondary windings disposed in series between the source and load wherein the transformers generate different voltage level changes in the voltage between the source and the load. Although permitting some improvements in voltage resolution and range, this conventional device has several drawbacks. First, the device includes three primary windings in each transformer-one for the addition of voltage, one for subtraction of voltage and one for a shorted (null) voltage. The large number of windings increases the component cost and reduces its reliability and requires sizing the transformers to handle relatively high power requirements. Second, current delivered to the primary windings of each transformer had to be of the same polarity or a null polarity limiting the ability to obtain sufficient resolution of voltage levels without a large number of transformers.
The inventors herein have recognized a need for a voltage regulator that will minimize and/or eliminate one or more of the above-identified deficiencies.